Field of the Invention
The present invention is directed to controllers and more particularly to compensation circuits which react to load changes.
There are a wide variety of controllers used to supply power to various types of loads. Controllers used to supply power to multiple lamps may include constant current or constant voltage types of controllers. Problems are encountered, however, when one lamp of a parallel connected group of lamps fails. The decrease in voltage drop across the line when a lamp fails increases the voltage across the remaining lamps. A constant voltage source, because it is incapable of recognizing that one of the lamps has failed, continues to supply the same voltage to the line plus the lamp load, resulting in a decreased voltage drop across the line resistance and an increased voltage across the lamp load. This may cause a premature failure of the remaining lamps.
A constant current source suffers from an even greater problem. When a lamp of a parallel connected group of lamps fails, the current drawn by the lamp group decreases. The constant current source, sensing a drop in current, increases its output voltage to maintain a constant current output. In that situation, the remaining lamps must dissipate the additional power resulting from the increase in the voltage output from the controller.
There is a need for a lamp controller capable of recognizing when one lamp of a parallel connected group of lamps fails. In response to that lamp's failure, the voltage output from the controller should be reduced so that the remaining lamps do not fail prematurely.
Another problem which prior art controllers suffer from is thermal drift. It is well known that the output of various electrical components drifts as the temperature of the component changes. The typical prior art solution is to provide an additional circuit element such as a thermistor which has a similar thermal drift but in the opposite direction. The problem with supplying thermistors is that the thermistor must be carefully matched to the thermal drift of the element for which compensation is required. Where there is a line of related products, it is often necessary to have a special thermistor for each product. This results in increased spare parts inventory as well as increased maintenance cost.